U.S. patent application number 11/722482 was filed with the patent office on 2010-03-11 for sensor system for an in-line inspection tool.
This patent application is currently assigned to PII LIMITED ATLEY WAY. Invention is credited to Peter Couchman.
Application Number | 20100060273 11/722482 |
Document ID | / |
Family ID | 34113084 |
Filed Date | 2010-03-11 |
United States Patent
Application |
20100060273 |
Kind Code |
A1 |
Couchman; Peter |
March 11, 2010 |
SENSOR SYSTEM FOR AN IN-LINE INSPECTION TOOL
Abstract
In an in-line pipe inspection tool, sensors for inspecting the
pipe are mounted on sensor blocks moveable relative to the body of
the tool. However, when the sensor blocks move radially to conform
to different pipe diameters, the circumferential distances between
the sensors changes. To ameliorate the effect of this, the sensor
blocks have a shape such that one axial edge of each sensor block
circumferentially overlaps the opposite edge of an adjacent sensor
block. With such an arrangement, when the sensor block are
operating at minimum diameter, part of one sensor block will
overlap an adjacent block, in the circumferential direction. As the
diameter of the pipeline in which the pig is used increases, the
degree of overlap will reduce, and may even reduce to zero, but
there will still be no overall axial gaps between the sensor
blocks. Thus, by suitable shaping of the sensor blocks the tool can
be used with a wide range of pipe diameters.
Inventors: |
Couchman; Peter;
(Newcastle-upon-Tyne, GB) |
Correspondence
Address: |
STITES & HARBISON PLLC
1199 NORTH FAIRFAX STREET, SUITE 900
ALEXANDRIA
VA
22314
US
|
Assignee: |
PII LIMITED ATLEY WAY
CRAMLINGTON
GB
|
Family ID: |
34113084 |
Appl. No.: |
11/722482 |
Filed: |
December 5, 2005 |
PCT Filed: |
December 5, 2005 |
PCT NO: |
PCT/GB05/04657 |
371 Date: |
October 23, 2009 |
Current U.S.
Class: |
324/220 |
Current CPC
Class: |
G01N 27/83 20130101;
G01N 27/902 20130101; F16L 55/28 20130101 |
Class at
Publication: |
324/220 |
International
Class: |
G01N 27/72 20060101
G01N027/72 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2004 |
GB |
0428127.5 |
Claims
1. An in-line pipe inspection tool having: a body; magnets mounted
in said body for generating a magnetic field in a pipe; and a
plurality of sensor blocks, each sensor block supporting at least
one sensor for sensing said magnetic field; wherein said sensor
blocks are arranged adjacent to each other in an array, the array
extending around at least a part of the circumference of the body,
each sensor block is movable in a radial direction relative to said
body, and each sensor block in the array is shaped such that one
axial edge of each sensor block circumferentially overlaps the
opposite axial edge of an adjacent sensor block, at least when the
sensor blocks are in a radially innermost position.
2. An in-line pipe inspection tool according to claim 1, wherein
each of said sensor blocks supports a plurality of sensors.
3. An in-line pipe inspection tool according to claim 1, further
including a carrier, wherein the sensor blocks are connected to the
carrier by a flexible linkage.
4. An in-line pipe inspection tool having: a body; magnets mounted
in said body for generating a magnetic field in a pipe; and a
plurality of sensors for sensing said magnetic field; wherein said
sensors are mounted on sensor blocks, which sensor blocks are
arranged adjacent to each other in an array, the array extending
around at least a part of the circumference of the body, and the
array includes a carrier on which the sensor blocks are mounted,
the carrier being of a circumferential length which is variable,
thereby to vary the circumferential spacing between the sensor
blocks.
5. An in-line pipe inspection tool according to claim 4, wherein
said carrier comprises a plurality of support parts connected by a
deformable link said sensor blocks being mounted on said support
parts.
6. An in-line pipe inspection tool according to claim 4 wherein
said body comprises a plurality of moveable parts, movement of said
parts causing said varying of the circumferential length of said
carrier.
7. An in-line pipe inspection tool according to claim 1, wherein
that one axial end of each of said sensor blocks is
circumferentially offset relative to the other axial end of the
corresponding one of said sensor blocks.
8. An in-line pipe inspection tool according to claim 7, wherein an
intermediate part of each of said sensor blocks is
circumferentially offset relative to said one axial end in the
opposite direction to the offset of said intermediate part relative
to said other axial end.
9. An in-line pipe inspection tool according to claim 1, wherein
the sensor blocks are triangular, with alternating sensor blocks
having their apexes pointing in opposite axial directions.
10. An in-line pipe inspection tool according to claim 1, wherein
the sensor blocks are T shaped, with the left of the T of
alternating sensor blocks pointing in opposite axial directions.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a sensor system for an
in-line pipe inspection tool. Such an in-line pipe inspection tool
is also known as a pipeline pig.
[0003] 2. Summary of the Prior Art
[0004] It is known to inspect a pipeline from the inside using a
pipeline pig which passes down the pipe. For magnetic inspection,
the pig has permanent magnets defining pole pieces, which are
positioned adjacent to the inner wall of the pipe. Those magnets
then generate magnetic fields which magnetise the wall of the pipe.
Sensors are provided between the magnetic poles, which detect the
magnetic flux density at the internal surface of the pipe. The
magnetic field in the pipe wall is normally constant, but is
disturbed by a flux changing feature, such as a defect, weld bead
or wall thickness change, and magnetic flux then leaks out of the
pipe at such a feature, to be detected by the sensors. As the
pipeline pig is driven along the pipe, the location of the pole
pieces, and the sensors, moves along the pipe enabling the internal
surface of the pipe to be inspected.
[0005] In known arrangements, the sensors may be mounted directly
to the body of the pig, but it is more usual to mount them on a
sensor carrier, which may also carry the pole pieces of the
magnetic pole, and which sensor carrier is connected to the body of
the pig by a deformable linkage. Such a deformable linkage permits
the sensor carrier to move radially, to allow it to pass e.g. a
deformation in the pipe. Thus, the sensor carrier usually forms a
body conforming to part of the circumference of a cylinder, and the
sensors are mounted on the carrier so as to extend around the
circumference of the part-cylinder. Thus, as the pipeline pig moves
along the pipeline, the sensors sense an arc of the pipeline, at a
given position along the pipeline. Normally, a plurality of such
sensor carriers, each with a plurality of sensors, are provided on
the pipeline pig, so that the whole circumference of the pipeline
can be sensed.
SUMMARY OF THE INVENTION
[0006] It is desirable that pipeline pigs may be used in pipelines
of a range of diameters, and therefore the sensors are normally
mounted in sensor blocks, each of which contains one or a plurality
of sensors, and which sensor blocks are themselves mounted on
supports, often referred to as fingers, which permit the sensor
blocks to move radially relative to the body of the pig. The radial
movement of the sensors permit the sensor blocks to conform to
pipelines of different diameters, at least over a range of such
diameters.
[0007] However, where the sensor blocks move radially, it will be
appreciated that the circumferential distance between the sensor
blocks will then change. The minimum diameter of the pipeline in
which the pig can be used then corresponds to that diameter for
which the sensor blocks will be in contact with each other, in the
circumferential direction. As such a pig is used with larger
diameter pipelines, the sensor blocks, and hence the sensors of
adjacent blocks, will move apart. This has the problem that the
sensing action may be less than optimal for many pipeline diameters
for which the pig is intended for use. Attempts have been made to
overcome this problem by having more than one sensor block ring,
with the sensor rings being axially displaced along the body of the
pig. Note that in such arrangements, although we have referred to
the sensor blocks being in a ring, they may be in one or more arcs
which need not extend all the way around the body.
[0008] Therefore, the present invention seeks to provide a sensor
arrangement which can adjust to different diameters. At its most
general, a first aspect of the present invention proposes that the
sensor blocks have a shape such that one axial edge of each sensor
block circumferentially overlaps the opposite edge of an adjacent
sensor block. That overlapping should occur at least in the radial
innermost position of the radial movement of the sensor blocks and
possibly may occur over the whole movement range of the sensor
blocks. Effectively, any gap between adjacent sensor blocks, at
least in the radial innermost position, has ends which are not
aligned, so that one axial end of such a gap is displaced
circumferentially relative to the other end.
[0009] With such an arrangement, when the sensor block are
operating at minimum diameter, part of one sensor block will
overlap an adjacent block, in the circumferential direction. In
such a situation, there is no effective overall circumferential gap
between the sensor blocks, when the sensors are viewed as a whole,
because although there is a circumferential gap at any axial
position, there is no direct axial path between the sensor blocks.
As the diameter of the pipeline in which the pig is used increases,
the degree of overlap will reduce, and may even reduce to zero, but
there will still be no overall axial gaps between the sensor
blocks. Indeed, it would still be possible for the sensor blocks to
move further apart, and thus have a small overall axial gap,
without significant problem.
[0010] Thus, a first aspect of the present invention may provide an
in-line pipe inspection tool having: [0011] a body; [0012] magnets
mounted in said body for generating a magnetic field in a pipe; and
[0013] a plurality of sensor blocks, each sensor block supporting
at least one sensor for sensing said magnetic field; [0014] wherein
said sensor blocks are arranged adjacent to each other in an array,
the array extending around at least a part of the circumference of
the body, each sensor block is movable in a radial direction
relative to said body, and each sensor block in the array is shaped
such that one axial edge of each sensor block circumferentially
overlaps the opposite axial edge of an adjacent sensor block, at
least when the sensor blocks are in a radially innermost
position.
[0015] It should be noted that, in the structure defined above, the
terms axial, radial, and circumferential are defined relative to
the sensor body, which itself is shaped so as to pass down the
pipe, and therefore those terms also normally refer to directions
within the pipe, at least when the tool is in use. Moreover, the
reference to circumferential overlapping refers to overlapping when
viewed in the axial direction. It thus does not require part of one
sensor block to overlap another in the radial direction.
[0016] As in the known arrangements, each sensor block may support
only one sensor, but it is preferable that plurality of the sensors
are mounted on each block. It is then possible to mount the sensors
so that there is no significant circumferential gap between
circumferentially successive sensors, at least over a wide range of
possible radial movement of the sensor blocks.
[0017] Preferably, each sensor block in the array is shaped that
one axial end of each sensor block is circumferentially displaced
relative to the other axial end of that sensor. Effectively, the
longitudinal axes of the sensor blocks are then inclined relative
to the axis of the pig. Then, the circumferential overlapping of
adjacent blocks is achieved because the circumferential
displacement of the respective axial ends of each sensor means that
adjacent sensors have one axial end of one sensor aligned with the
opposite axial end of the adjacent sensor.
[0018] Preferably an intermediate part of the sensor block is
displaced relative to one axial end in the opposite direction from
the displacement of the other axial end. Such an arrangement will
mean that there will be a circumferential overlap between each
sensor block and the sensor blocks on either side. This further
assists to minimise the effective axial separation of the
sensors.
[0019] However, other sensor block arrangements are possible, for
example, the sensor blocks may be generally triangular, with
alternate sensor blocks having their apexes pointing in opposition
axial directions. Note that, with such triangular sensor blocks,
the corners of the triangles may be omitted.
[0020] Yet a further possibility is that the sensor blocks are "T"
shaped with the legs of alternate "T" shaped sensor blocks pointing
in opposite axial directions, so that the arms of adjacent "T"
shapes overlap in the circumferential direction.
[0021] Normally, as in known arrangements, the sensor blocks will
be mounted on a carrier via flexible supports, often known as
fingers, which allow the sensor blocks to move radially, but
maintain their orientation relative to the axis of the pig. For
this purpose, the sensor blocks may be mounted on the carrier via
sprung or parallelogram linkages. Moreover, as in the known
arrangements, the carrier may extend around only a part of the
circumference of the pig. It is then possible for there to be
multiple sensor arrays, each with its own carrier.
[0022] In the first aspect of the invention, discussed above,
changes in pipeline diameter are accommodated by radial movement of
the sensor blocks. However, the mounting of the sensor blocks on
the body, or on a carrier, will limit the amount of radial movement
that is possible. It may be necessary for the pig to accommodate
large changes in pipeline diameter. Therefore, a second aspect of
the invention proposes that sensors are mounted on a carrier, the
circumferential length of which is variable. Moreover, it is
preferable in such an arrangement that the varying of the
circumferential length of the carrier causes a variation in the
effective radius of curvature of the carrier. Thus, the carrier
itself adapts to different diameters of pipeline, moving the
sensors to an appropriate position relative to the pipeline
wall.
[0023] Whilst this second aspect may be used in conjunction with
the first aspect, it can also be used independently.
[0024] Thus, the second aspect of the present invention may provide
an in-line pipe inspection tool having:
[0025] a body;
[0026] magnets mounted in said body for generating a magnetic field
in a pipe; and
[0027] a plurality of sensors for sensing said magnetic field;
[0028] wherein said sensors are mounted on sensor blocks, which
sensor blocks are arranged adjacent to each other in an array, the
array extending around at least a part of the circumference of the
body, and the array includes a carrier on which the sensor blocks
are mounted, the carrier being of a circumferential length which is
variable, thereby to vary the circumferential spacing between the
sensor blocks.
[0029] There are several different ways of achieving a variation in
the carrier circumferential length. For example, it is possible to
provide a carrier consisting of the support parts, to which the
sensor blocks are connected, with the support parts being
themselves interconnected by a deformable link. Then, the support
parts may be moved apart, but the deformable link ensures that they
maintain appropriate spacing, thereby controlling the spacing of
the sensor blocks. Such an arrangement has the advantage that it is
also readily possible to vary the effective radius of the carrier.
In such an arrangement, the carrier may also comprise plurality of
strips which are moveable relative to the other, and act as a
backing for the flexible connections. This ensures that the carrier
can have sufficient rigidity.
[0030] In such an arrangement, the carrier needs to be driven to
move circumferentially. One way to achieve this is by having a pig
whose body has moveable parts, to adapt to different diameters. As
the parts of the body move the carrier is lengthened or
shortened.
[0031] An embodiment of the present invention will now be described
in detail, by way of example, with reference to the accompanying
drawings, in which:
[0032] FIG. 1 is a view of part of the body of an in-line pipeline
inspection tool, having an array of sensors in accordance with
present inventors;
[0033] FIG. 2 shows two of the sensors of the embodiment of FIG. 1,
in a first position;
[0034] FIG. 3 shows the sensors of FIG. 2, in a second
position;
[0035] FIG. 4 is a view similar to FIG. 1, but in which the overall
circumference of the body has been increased and the
circumferential length of the array of sensors increased;
[0036] FIG. 5 is a view similar to FIG. 4, but with some of the
sensors removed;
[0037] FIG. 6 is a view similar to FIG. 5, but with all but the end
sensors, and their associated supports, being removed;
[0038] FIG. 7 is an end view of the body of the in-line pipe line
inspection tool, as in the position shown in FIG. 1;
[0039] FIG. 8 is an end view of the body, when the overall
circumference of the body has been increased as shown in FIG.
4;
[0040] FIGS. 9a and 9b illustrate an alternative sensor block
figuration;
[0041] FIGS. 10a and 10b show a further sensor block configuration;
and
[0042] FIGS. 11a and 11b show yet a further sensor block
configuration.
[0043] Referring first to FIG. 1, an in-line pipe inspection tool
(pipeline pig) has a body 10 with an array of sensor blocks 12
thereon. Each sensor block 12 is connected via a connection 13 to a
support 14, which supports are interconnected by a flexible
connection 15. Each sensor block 12 is pivotally linked to the
connection 13, and each connection 13 is pivotally connected to the
corresponding support 14. Thus, the sensor blocks 12 may move
radially relative to the body 10 by movement of the connection 13.
Moreover, the sensor blocks 12 are able to maintain a fixed
orientation relative to the longitudinal axis of the body 10, due
to an additional linkage that will be described later.
[0044] FIG. 2 then shows two of the sensor blocks 12 in more
detail. It can be seen that each sensor block 12 has a first end
12a at one axial end of the sensor block, which end 12a extends
generally perpendicular to the longitudinal axis of the body 10. An
intermediate part 12b extends at an angle inclined to the ends 12a,
a further intermediate part 12c then extends generally parallel to
the longitudinal axis of the body 10, and there is a further
inclined part 12d, but inclined in the opposite direction to the
part 12b, terminating in an end 12e at the opposite axial end of
the sensor 12 from the end 12a. It can been seen that the ends 12e
are circumferentially offset relative to the ends 12a, because of
the inclination of the parts 12b. This means that the gap 20
between the sensor blocks 12 follows a convoluted path and there is
a region 21 in which the sensors 12 overlap in the circumferential
direction. Thus, there is no overall gap between the sensor blocks
12, even though there is a gap at any axial position, because there
is no direct axial path between the sensor blocks, when in the
position shown in FIG. 2.
[0045] FIG. 2 also illustrates that each sensor block 12 carries 6
sensors 12f, three at each end of the sensor block 12. When the
sensor blocks 12 are in the position shown in FIG. 2, the sensors
12f at one axial end of one sensor block 12 overlap the sensors 12f
at the opposite end of the adjacent sensor block 12. In the
position shown in FIG. 3, there is no such overlap, but
nevertheless the spacing between the end most sensors of adjacent
sensor blocks is not significantly greater than the spacing of the
sensors 12f within a sensor block 12.
[0046] As the sensor blocks 12 move radially outwards, due to
pivoting of the links 13, their increased radial separation from
the longitudinal axis of the body 10 will mean that they move
apart. Thus, as shown in FIG. 3, if the sensor blocks are at an
increased radial distance from the longitudinal axis of the body
10, the gap 20 will widen. However, even in the position shown in
FIG. 3, there is still an overlap 21 between the sensor blocks 12,
although that overlap 21 is reduced as compared with the sensor
blocks position shown in FIG. 2. Thus, it would be possible for the
sensor blocks to move further apart in the view of FIG. 3, before
there was an overall gap between them, i.e. there was no overlap
21. Thus, the use of the sensor blocks 12, which are of convoluted
shape, generates the overlap 21 and this improves the performance
of the sensors because all parts of the circumference of the
pipeline corresponding to the arc of the sensors will be
sensed.
[0047] In this embodiment, not only can the sensor blocks 12 move
radially, to increase their circumferential separation, but the
sensor carrier formed by the supports 14 and the flexible strip 15
is itself of variable circumferential length. In this embodiment,
the pig body 10 is deformable, to change its outer diameter. Thus,
as illustrated in FIG. 4 two parts 10a, 10b of the body 10 move
radially, to create a gap 10c, and in doing so increase the
circumference of the body. To compensate for this, the carriers
supporting themselves move circumferentially, so that the array of
sensors has an increased circumferential length, but the flexible
strip 15 deforms by the same amount between each support 14,
thereby ensuring that the spacing between the supports 14, and
hence the sensor blocks 12, is maintained uniform.
[0048] FIG. 5 is a similar view to FIG. 4, but with some of the
sensor blocks 12 removed, to enable the supports 14 to be seen more
clearly. In particular, FIG. 5 shows that there may be a further
flexible strip 16 at or adjacent to the opposite ends of the
supports 14 from the strip 15, to prevent twisting of the supports
14 relative to the longitudinal axis of the body 10. FIG. 5 also
shows that there may be a mounting strip structure 30 linking the
supports 14 to provide the support structure for the one of sensor
blocks. This strip structure is shown mare clearly in FIG. 6, which
is a view similar to FIG. 5, but with all but the end sensor blocks
12 of the array removed. It can be seen that the mounting strip
structure 30 comprises two overlapping strip parts 30a, 30b which
are fixed to the end supports 14 of the array, and can slip one
relative to the other in the circumferential direction. The strip
parts 30a, 30b provide structural support for the arc of sensor
blocks. The strips 15, 16 then ensure even circumferential spacing
between the supports 14.
[0049] FIG. 6 also shows that there may be a further flexible
linkage 17 between the sensor blocks 12 and the support 14, the
purpose of which is to provide, together with the link 13, a
parallelogram linkage between the supports 14 and the sensor blocks
12, so that the sensor blocks 12 maintain the correct orientation
relative to the longitudinal axis of the body 10. Note that
although the linkages 17 are connected between the respective
sensor blocks 12 and the supports 14, for the sensor blocks shown
in FIG. 6, they will be connected between the sensor blocks 12 and
the support 14 for the other sensor blocks, which are omitted from
the view of FIG. 6, since those supports 14 will move relative to
the body.
[0050] As has previously been mentioned, the body 10 has parts
which move radially to increase the circumference of the body. This
is illustrated in more detail in FIGS. 7 and 8. As shown in FIGS.
7, the body 10 has 5 body parts 10a, 10b, 10d, 10e and 10f, mounted
on a central core 40, with the body parts, 10a, 10b, 10d, 10e and
10f being movable radially relative to that core 40 to the position
shown in FIG. 8. There are then linkages 41a, 41b, 41d, 41e and 41f
between the one 40 and the levelling parts 10a, 10b, 10d, 10e and
10f to achieve such movement. Note also that the body parts 10a,
10b, 10d, 10e and 10f of the body 10 need not extend over the full
axial length of the body 10, so that there may be another part 42
of the body with no such radial movement.
[0051] It is also possible for the body parts 10a, 10b, 10d, 10e
and 10f to move radially by different amounts along their length.
The body parts can then adopt a conical shape. However, it is still
desirable that the arc of sensor blocks remains uniform and aligned
with the wall of the pipe, and this can be achieved as a result of
pivots at the mounting points of the sensor block assembly.
[0052] It should be noted that the arrangement of the body parts
10a, 10b, 10d, 10e, and 10f discussed above is described in more
detail in U.S. Pat. No. 6,538,431 and will not be discussed in
further detail now.
[0053] In the arrangements which have been described, the sensor
blocks have a convoluted shape in which the first axial end 12a is
circumferentially offset relative to the opposite axial end 12e.
However, the present invention is not limited to such a block
arrangement. FIGS. 9 to 11 show alternative configurations for the
sensor blocks.
[0054] In the arrangement shown in FIGS. 9a and 9b, the sensor
block 50 has a general triangle shape, although the corners of the
angles have been removed so they are not needed). Thus, the sensor
blocks 50 have an edge 51 which is generally perpendicular to the
axis of the body 10, hence to the point, an inclined sides 52.
Alternate sensor blocks 50 point in opposite directions, so that
the edges 52 of adjacent sensor blocks 50 are parallel. Thus, at
the inner most position of the sensor blocks, narrow gaps 53 are
defined between adjacent sensor blocks, which gaps 53 are inclined
to both the axial and circumferential directions. Moreover, sensors
54 are mounted on the sensor blocks 50, arranged along the inclined
edges 52.
[0055] Thus, in the innermost position of the sensor blocks 50,
shown in FIG. 9a, the edge 50 of one sensor block overlaps, in the
circumferential direction, the edge 50 of the adjacent sensor
block. Thus, in that position, the circumferential spacing of at
least one sensor in one block relative to at least one sensor in
the adjacent block is not greater than the spacing between the
sensors within each block. Thus, sensing can be carried out over
the four circumferential extent of the sensor block array. This is
true even when the sensor blocks over part to the position shown in
FIG. 9b. The width of the gaps 53 has increased, and indeed in the
position shown in FIG. 9b the sensor blocks 50 have moved so that
their edges 51 no longer overlap. Nevertheless, the gap 56 between
the end most sensors of adjacent sensor blocks is not significantly
greater than the gap 56 between adjacent sensors within a block, so
that substantially uniform sensing can still be achieved when the
sensor blocks 50 are in the position shown in 9b, corresponding to
the greater remedial position than that shown in FIG. 9a.
[0056] FIGS. 10a and 10b illustrate another sensor arrangement, in
which the sensor blocks 60 are generally T-shaped, with the legs 61
of adjacent blocks 60 extending in opposite axial direction. Thus,
the arms 62 of the sensor block 60 overlap in the circumferential
direction, when the sensor blocks 60 are in the inner most radial
position. The gap 63 between adjacent sensor blocks is convoluted,
with one axial end of each gap 63 being displaced relative to the
other axial end.
[0057] As shown in FIGS. 10a and 10b, sensors 64 are mounted on the
arms 62 and the leg 61 of the sensor block 60. Thus, in the inner
most position shown in FIG. 10a, the circumferential spacing
between at least one sensor of each adjacent sensor blocks is less
than the circumferential spacing of the sensors within each sensor
block. Hence, even when the sensor blocks 60 move radially, so
increasing their circumferential separation, as shown in FIG. 6b,
the circumferential separation 65 between the end most sensor 64 of
adjacent sensor blocks 60 is not significantly greater than the
spacing 66 between adjacent sensors within the sensor block.
[0058] FIGS. 11a and 11b illustrate a modification of the
arrangement of FIGS. 10a and 10b, in which the sensors 64 are
mounted only on the arms of the "T-shaped" sensor blocks 60. The
arrangement of FIGS. 11a and 11b is otherwise the same as that of
FIGS. 10a and 10b, and will not be described in further detail. The
same reference numerals are used to indicate corresponding
parts.
* * * * *